Review of the Prior Art
Tension-leg offshore platforms are known. Such platforms are useful in the drilling of offshore oil and gas wells, but are most commonly used as facilities for the production of oil and gas wells drilled at subsea locations by use of other equipment and techniques. Tension-leg platforms are useful in waters ranging from shallow (300 foot depths or less) to deep (1000 foot depths or more), but are used to best advantage in deep waters where the economics of rigid towers built on the sea floor become unattractive.
Tension-leg platforms are connected to the seafloor. They have operations areas located above the ocean surface. The legs which connect the surface portions of the platform to the sea floor are loaded in tension by positive buoyancy of the surface portions of the tower; this is in contrast to rigid towers in which the supporting legs are loaded compressively as columns.
Tension-leg platforms described to date are one-of-a-kind structures; such structures are known to be very costly. Also, such structures do not have components which are rapidly reusable in other places after drilling and production activities at their initial locations have been completed. Previously described tension-leg platforms entail expensive installation equipment located either on the platform itself or on other special purpose support and installation vessels. A need exists for tension-leg platforms having reusable components and which can be installed with minimal use of costly equipment.
U.S. Pat. No. 4,297,965 discloses a tension leg structure comprising a single pipe structure anchored to the ocean floor and connected to a floating platform by a plurality of wire ropes. Such wire ropes are spaced apart by centers which are at least ten feet apart.
An aspect of this invention is the recognition and solution of a previously unrecognized problem inherent in prior tension leg structures for tension leg platforms. The problem is present in the leg structure described in U.S. Pat. No. 4,297,965. When installed, the wire ropes are equally tensioned. During operation, however, currents and wave action tend to deflect the platform laterally away from a position centered over the anchor. Such lateral shifting of the platform can approach 10% of the water depth. When the platform moves off center, the distance between the top of the pipe and the platform changes for each wire rope so that the distances become generally unequal. Consequently one wire rope will absorb most or all of the tension while the remaining ropes in the set will tend to become slack. Such uneven tensioning increases wire rope fatigue and must reduce the useful operating life of the tension leg assembly. Stated differently, when a tension leg structure comprises a single pipe connected to a platform by a group of parallel spaced wire ropes, one of the ropes will almost always be effectively shorter than its counterparts and the cables will be unevenly loaded.
There is need for a tension leg structure which is not afflicted by this problem of uneven loading of cables.